Motor vehicle wheel suspension

ABSTRACT

A motor vehicle wheel suspension has a compliance device 23 which permits limited longitudinal displacement of the wheel relative to the vehicle so that the wheel can be displaced forwardly and rearwardly by rotating out of balance forces when the wheel rotates at a critical wobble speed- and the geometry of the suspension pivot points 3,9,18 and 15 is arranged to cause the wheel to toe in for forward longitudinal displacement of the wheel and to toe out for rearward longitudinal displacement of the wheel, the amount of toe in or toe out for a given longitudinal displacement being matched with the toe in and toe out arising from wheel oscillations at the critical wobble speed to reduce the effect of the out of balance forces on the unsprung part of the vehicle.

DESCRIPTION

This invention relates to motor vehicle wheel suspensions, and is ofparticular utility in independent steerable front wheel suspensions.

A major problem with all front wheel suspensions is known in theindustry as "shimmy". This is caused inter alia by the front wheels or afront wheel being out of balance. In the remainder of this specificationreference will be made to one wheel, it being understood that similarconsiderations apply to the corresponding road wheel at the other sideof the vehicle. Rotation of an out of balance wheel creates an out ofbalance force directed radially of the wheel and rotating with thewheel. At certain critical vehicle speeds, hereinafter called "criticalwobble speed", typically about 25 m.p.h., about 55 m.p.h., and about 85m.p.h., a resonant condition occurs and the wheel oscillates in acertain mode characteristic of the suspension. The base frequency ofthis oscillation is of the order of 10 Hz with harmonics atapproximately 5 and 18 Hz. The lower the frequency the greater theamplitude of the oscillation; and the higher the frequency the lower theamplitude. In most known front suspensions, these oscillations aretransmitted through a steering linkage and steering mechanism to thesteering wheel, and this causes discomfort to the driver. In order toavoid this problem, it is known to carefully balance the wheels but thissolution requires additional operation during vehicle manufacture andrebalancing of the wheels in service as the tires wear or are replacedor the balancing weights are lost.

It is therefore an object of the present invention to provide animproved vehicle wheel suspension which will be tolerant of wheelimbalance so that wheel balancing is either avoided completely or isnecessary less frequently or is required to less accurate standards.

We have realised and confirmed by experiment and calculation that thewheel oscillations caused by the rotating out of balance forces are oflimited amplitude even if the wheel is substantially free to movelongitudinally of the vehicle. As the wheel is displaced forwardly bythe unbalance force it tends to turn inwardly (toe in) and as the wheelis displaced rearwardly it tends to turn outwardly (toe out), becausethe unbalance forces when acting forwardly or rearwardly have a turningmovement about the king pin axis of the wheel. The magnitude of toe inand toe out movement caused by the out of balance forces is greater insuspensions with a large offset from the king pin axis.

The present invention utilises a novel compliance device in combinationwith appropriate modification of the geometry of the suspension toreduce, not the oscillations, but the transmission of them to thesteering wheel or in the case of a rear suspension, to the body.

It is well known to fit a "compliance device" in the suspension of amotor vehicle. This device provides a resilient connection of the wheelto the vehicle and is intended to reduce noise, vibration and harshnesstransmitted to the sprung part of the vehicle from the road surface.

The novel compliance device of the invention permits limited forward andrearward displacement of the wheel spindle relative to the vehicle. Itslongitudinal resistance for such limited forward and rearwarddisplacement must be proportionately less than that required to overcomethe friction in the steering gear linkage.

Hitherto the steering link and suspension arm have in many vehiclesapproximately constituted a parallelogram (that is the pivot points ofthe steering link and suspension arm and their respective connections tothe wheel are at the four corners of the parallelogram) so that as thewheel is displaced longitudinally as permitted by the compliance devicethe road wheel is not turned. In some vehicles the steering link andsuspension arm have not constituted a parallelogram. This has not beento achieve any special result but has been dictated by packagerequirements.

In the case of rear wheel suspensions, the oscillations of the rearwheels due to the wheels being out of balance are resisted by the rearsuspension. This results in body shake. The invention may be used in arear wheel suspension as well as in a front wheel suspension.

A motor vehicle suspension has the following features: a compliancedevice permits limited longitudinal displacement of the wheel relativeto the vehicle so that the wheel can be displaced forwardly andrearwardly by rotating out of balance forces when the wheel rotates at acritical wobble speed; and the geometry of the suspension is such thatit causes the wheel to toe in for forward longitudinal displacement ofthe wheel and to toe out for rearward longitudinal displacement of thewheel by substantially the same amount for a given longitudinaldisplacement as the toe in and toe out movements which would be causedfor the same longitudinal displacement at said critical wobble speed bysaid out of balance forces in the absence of any restraint of angularmovements of the wheel about a substantially vertical axis, whereby theeffect of the out of balance forces on the unsprung part of the vehicleis reduced.

In the case of steerable independent front wheel suspension, the effectis that the transmission of wheel oscillations to the steering wheel isreduced or even eliminated; and in the case of rear suspensions the bodyshake caused by out of balance forces is reduced.

Somewhat inaccurately but possibly more clearly the invention may besummarised as follows: an out of balance force on the road wheel causesit to oscillate at a high frequency about a vertical axis (the kingpin); the out of balance force exerts a forward or rearward longitudinalforce on the suspension linkage which causes the wheel to move aspermitted by the compliance device forwardly or rearwardly relative tothe sprung part of the vehicle; and the idiosyncratic geometry of thesuspension arm and the steering link is such that when this oscillationtakes place the steering link is displaced substantially at right anglesto its axis so that axial displacement of the steering link which wouldotherwise occur and cause movement of the steering wheel is reduced oreliminated.

"Shimmy" occurs at different frequencies. The energy of the oscillatingwheel and the longitudinal forces exerted on the wheel by the out ofbalance forces is greater at higher critical wobble speeds. Theamplitude on the other hand is lower at higher critical wobble speeds.Therefore if the longitudinal movement permitted by the compliancedevice was independent of the frequency of oscillation the compliancedevice would provide more movement than was necessary at the higherfrequencies. This is undesirable because the vehicle speed is so high atthe higher frequencies that good control of suspension movements isrequired for handling purposes.

In order to overcome this problem the compliance device is formed withresilient material which becomes harder and permits less displacement asthe frequency of the road wheel oscillations increases. In effecttherefore a degree of progressive change in the compliance device isprovided to match it approximately to the amplitude of the "shimmy"oscillations.

The invention is particularly suitable for use with Macpherson frontsuspensions, in which the compliance device is used to connect thesuspension arm to a stabiliser (or anti-roll) bar or tie rod. Theinvention can however be incorporated in other forms of suspensions forexample the double wishbone or short long arm suspension. The term"suspension arm" includes the two links of the double wishbone orshort/long arm suspensions.

The invention is hereinafter particularly described with reference tothe accompanying drawings, in which:

FIG. 1 is a plan view of the Macpherson suspension incorporating theinvention;

FIG. 2 is a front elevation of the suspension;

FIG. 3 is a diagramatic plan view of the geometry of a conventionalfront suspension having a compliance device for suppression of noise,vibration and harshness;

FIG. 4 is a diagramatic plan view of the modified geometry of the frontsuspension incorporating this invention;

FIG. 5 is a section through part of the suspension of this invention;

FIG. 6 is a graph of the characteristics of a conventional compliancedevice and a compliance device according to this invention; and

FIG. 7 is a perspective view of a rear suspension which includes thisinvention.

The independent front suspension shown in FIG. 1 is of the type known asa Machpherson suspension. It comprises at each side of the vehicle asuspension arm 1 pivotally mounted in the sprung part of the vehicleabout a generally longitudinal axis 53 by a pivot bush 3 which permitslimited forward and rearward movement of the outer end 5 of thesuspension arm 1.

A wheel spindle 7 is connected by a suspension ball joint 9 to a kingpin carried by the suspension arm 1 and is fixed to the lower end of aconventional suspension strut 11. The upper end of the strut 11 is fixedto the sprung part of the vehicle by an upper mount 50 in theconventional way, the casing of the strut being rotatable relatively tothe piston of the strut so that the wheel spindle can rotate about aking pin axis defined by the upper mount 50 and the suspension balljoint 9. The wheel spindle 7 has a conventional stub axle 48 on which aroad wheel is rotatably mounted.

A steering arm 13 extends rearwardly from the wheel spindle 7 and isconnected by a steering ball joint 15 to a steering link 17. The latterin turn is connected by a ball joint 18 and a conventional rack andpinion steering mechanism to the steering wheel (not shown).

As the steering wheel is turned, the steering link 17 is moved along itsaxis. This turns the wheel spindle 7 about the king pin axis and soturns the road wheel.

Movement of the suspension arm 1 about the generally vertical axis iscontrolled by a U-shaped stabiliser (or anti-roll) bar 19. The base ofthe U is fixed in the sprung part of the vehicle at two spaced apartpositions 21 while each end of the bar 19 is connected to the suspensionarm 1 by a compliance device 23 shown in FIG. 5. The compliance deviceincludes resilient bushes 35 arranged to permit the outer end of thesuspension arm 1 a limited amount of substantially free forward andrearward movement, as described in more detail below.

Hitherto it has been conventional to arrange the centres of the steeringball joints 18 and 15, suspension ball joint 9 and pivot bush 3 so thatthey constitute a parallelogram. This is shown in FIG. 3. As the outerend of the suspension arm 1 moves forwardly or rearwardly as permittedby the elasticity of the compliance device 23, the toe in or toe out ofthe wheel is not affected. If the wheel is out of balance, the wheelwill oscillate about the king pin axis at one or more critical wobblespeeds of the vehicle. The oscillation occurs at a base frequency of 10Hz with harmonics at approximately 5 and 18 Hz in an average wheelassembly corresponding to vehicle speeds of about 25, 55 and 85 m.p.h.At lower frequencies the oscillation amplitude is greater than at thehigher frequency. But the kinetic energy of the oscillation varies withthe square of the velocity of the oscillations and hence is higher athigher frequencies.

These oscillations are known as "shimmy" or "nibble" and if they aretransmitted through the steering arm 13, steering link 17 and rack andpinion steering gear mechanism to the steering wheel they causeoscillation of the steering wheel.

According to this invention the geometry of the steering/suspension isaltered as shown in FIG. 4. The pivot points 3, 9, 18 and 15 no longerconstitute a parallelogram. This may be achieved in existing vehicles byrelocating the pivot bush 3 at the inner end of the suspension arm 1closer to the inner steering ball joint 18, or by relocating thesteering rack closer to the pivot bush 3. In some known vehicles theamount of relocation required may be of the order of 10 mm to 30 mm.Therefore when the wheel moves forwardly or rearwardly as permitted bythe compliance device 23, the geometry of the points 3, 9, 18 and 15 issuch that the suspension imparts a toe in to the road wheel with forwarddisplacement of the wheel relative to the vehicle and toe out withrearward displacement.

When the wheel is rotating at a critical wobble speed it oscillatesabout a generally vertical axis and there is a longitudinal forceexerted on the wheel and hence on the suspension arm 1. As the wheelmoves to the toe out position there is a rearward force on thesuspension arm; and as the wheel moves to a toe in position there is aforward force on the suspension arm.

The geometry of the suspension is such that it causes the wheel to toein for forward longitudinal displacement of the wheel and to toe out forrearward longitudinal displacement of the wheel by substantially thesame amount for a given longitudinal displacement as the toe in and toeout movements which would be caused for the same longitudinaldisplacement at said critical wobble speed by said out of balance forcesin the absence of any restraint of angular movements of the wheel abouta substantially vertical axis, whereby the effect of the out of balanceforces on the unsprung part of the vehicle is reduced.

At a critical wobble speed, the out of balance forces are such as tocause the suspension arm 1 to move as permitted by the compliance device23 and pivotal connection 3. This movement causes the suspensionsteering geometry to move the ball joint 15 at the outer end of thesteering link 17 to the position the steering link is being moved by theoscillations of the wheel. Consequently, there is no movement of thesteering wheel. It is of course difficult to establish the correctlocation of the pivot points 3 and 18 and to exactly match the path ofsteering ball joint 15 with the wheel oscillations. This however is theobjective and it is best achieved by calculations to establishtheoretically correct positions for the pivot points followed by finetuning of the suspension by testing the effect of a number of positionsclose to those established by theory.

The novel compliance device which is an important feature of thesuspension is shown in FIG. 5. The ends of the stabiliser bar 19 extendthrough an aperture in the suspension arm 1. The ends of the bar have areduced diameter portion 25, with a threaded end portion. A retainer 27engages the shoulder between the large and narrow diameter portions, andthe component parts are held between the retainer 27 and the retainer 29by a nut 31. A metal sleeve 33 is mounted on the part 25. A bearing 46fits an aperture in the arm 1 and transfers the loads from the arm 1 tothe stabiliser bar 19. Located either side of the bearing 46 are twobushes 35 made of micro-cellular polyurethane. These bushes provide theresilience in the compliance device to permit the arm 1 to move relativeto the end of the stabiliser bar 19 in the longitudinal direction of thevehicle. As a longitudinal force is exerted on the arm 1 by a wheel inthe state of "shimmy", it moves relative to the bar 19 against thebushes 35.

In FIG. 6, the y axis represents the load in kilo-newtons on a wheel inthe longitudinal direction of the vehicle, while the x axis representsdeflection of the compliance device in mm. Lines A1 and A2 represent theupper and lower limits for a typical load/deflection characteristic of aconventional compliance device; while curves B1 and B2 represent theupper and lower limits for the deflection characteristic for a car ofthe size of a Ford Cortina of the novel compliance device according tothis invention when the wheels are not oscillating due to shimmy.

The conventional compliance device has a linear load/deflectioncharacteristic. As can be seen in FIG. 6, the compliance device of theinvention offers little resistance to the initial 3 mm of deflection ineither direction. This is achieved by avoiding friction in the deviceand having very little preload on the bushes 35. The compliance deviceresiliently opposes deflection greater than 3 mm to control brakingloads and road shocks. Line 47 in FIG. 6 corresponds to the deflectionproduced by 0.8 g braking. Line 49 indicates the deflection which occursat a critical wobble speed due to wheel oscillations. The resistance ofthe compliance device to such deflections must be less than thefrictional resistance in the steering linkage to steering movements.

An important feature of this invention is that the deflectioncharacteristic of the compliance device 23 is not the same for allfrequencies of oscillation. A characteristic of micro-cellularpolyurethane is that the elasticity varies inversely with the frequencyof the change of load (such as occurs in a shimmy condition). Thus theresistance of the compliance device to deflection is greater at higherfrequency and hence the free movement allowed by the compliance deviceis less at a high critical wobble speed than at a low critical wobblespeed. When the high frequency lower amplitude oscillations occur theelasticity is less than when the lower frequency high amplitudeoscillations occur.

FIG. 7 shows an independent rear suspension incorporating the invention.It comprises a suspension arm 37 in the form of a wishbone, a stub axle39 pivotally connected to the arm 37 for movement about a longitudinalaxis of the vehicle, a coil spring 41 and a tie bar 43. The suspensionarm 37 is pivotally connected to the sprung part of the vehicle about agenerally longitudinal axis. The pivotal connection of the arm 37 to thevehicle is such that the outer end of the arm can move forwardly orrearwardly through a small distance. The camber of the wheel iscontrolled by a Macpherson strut 45 connected to the stub axle 39 andthe body. The tie bar 43 is pivotally connected to the sprung part ofthe vehicle for movement about a transverse axis, and is connected tothe suspension arm 37 by a compliance device 145 which is similar tothat described above with reference to FIGS. 5 and 6. When the wheeloscillates at a critical wobble speed, the rotating unbalance forcescause the wheel to move forwardly or rearwardly and the arm 37 to pivotabout the vertical axis; the geometry is such that this movement impartstoe in or toe out to the wheel in the same direction as the toe in ortoe out produced by the wheel oscillation.

I claim:
 1. A motor vehicle wheel suspension in which:a. a compliancedevice permits limited longitudinal displacement of the wheel relativeto the vehicle so that the wheel can be displaced forwardly andrearwardly by rotating out of balance forces when the wheel rotates at acritical wobble speed; and b. the geometry of the suspension is suchthat it causes the wheel to toe in for forward longitudinal displacementof the wheel and to toe out for rearward longitudinal displacement ofthe wheel by substantially the same amount for a given longitudinaldisplacement as the toe in and toe out movements which would be causedfor the same longitudinal displacement at said critical wobble speed bysaid out of balance forces in the absence of any restraint of angularmovements of the wheel about a substantially vertical axis, whereby theeffect of the out of balance forces on the unsprung part of the vehicleis reduced.
 2. A motor vehicle steerable independent front wheelsuspension in which:a. a road wheel is rotatable about a generallyvertical axis on a suspension link to steer the vehicle; b. thesuspension arm is pivotally mounted in the sprung part of the vehicleabout a generally longitudinal horizontal axis to permit suspensionmovement and about a generally vertical axis to permit limited forwardand rearward movement of the outer end of the arm; c. the steering wheelof the vehicle is connected to the road wheel by a steering link; d. thesuspension includes a compliance device which permits limited movementof the suspension arm about the generally vertical axis so that theouter end of the arm can be moved forwardly and rearwardly through asmall distance by rotating out of balance forces when the wheel rotatesat a critical wobble speed; and e. the geometry of the suspension armand the steering link is such that it causes the wheel to toe in forforward longitudinal displacement of the wheel and to toe out forrearward longitudinal displacement of the wheel by substantially thesame amount for a given longitudinal displacement as the toe in and toeout movements which would be caused for the same longitudinaldisplacement at said critical wobble speed by said out of balance forcesin the absence of any restraint of angular movements of the wheel abouta substantially vertical axis, whereby the effect of the out of balanceforces on the unsprung part of the vehicle is reduced.
 3. A suspensionas claimed in claim 1 or claim 2 in which the limited longitudinaldisplacement permitted by the compliance device is approximately equalto the amplitude in the longitudinal direction of the oscillations ofthe wheel at a critical wobble speed.
 4. A suspension as claimed inclaims 1 or 2 in which the resistance of the compliance device to saidlimited longitudinal displacement is less than the resistance of thesteering linkage to steering movements.
 5. A suspension as claimed inclaims 1 or 2 in which the compliance device resiliently controlslongitudinal displacements of greater amplitude than said limitedlongitudinal displacement.
 6. A suspension according to claims 1 or 2 inwhich the limited longitudinal displacement permitted by the compliancedevice is less for displacements at a first critical wobble speed thanat a second lower critical wobble speed.
 7. A suspension according toclaim 6 in which the compliance device includes bushes of amicrocellular material such as micro-cellular polyurethane arranged tolimit said longitudinal displacement.
 8. A suspension according toclaims 1 or 2 which is a Macpherson suspension and in which thecompliance device includes a resilient bushing connecting a stabilizerbar to the suspension arm.
 9. A suspension according to claim 8 in whichthe compliance device includes a bearing adapted to transfer the radialforce acting on the stabilizer bar to the suspension arm.
 10. Asuspension according to claim 1 which is a rear suspension having asuspension arm pivotally connected to the sprung part of the vehicleabout a longitudinal axis and about a vertical axis so that the outerend of the arm can move forwardly or rearwardly for said limiteddisplacement and a tie bar connected at one end to the suspension arm bythe compliance device and at the other end to the sprung part of thevehicle.
 11. A suspension according to claim 10 in which the compliancedevice resiliently controls longitudinal displacement of greateramplitude than said limited longitudinal displacement.
 12. A suspensionaccording to claim 11 in which the compliance device includes bushes ofa microcellular material such as micro-cellular polyurethane, arrangedto limit said longitudinal displacement.